The present invention relates to a procedure and means for controlling the d.c. motor of an elevator in an emergency braking, in which a current generated by the motor of the elevator during emergency braking is conducted into a braking resistance.
During emergency braking of an elevator, the braking proper, i.e. deceleration of the elevator cage, has to be managed as independently as possible of the other systems of the elevator. In the case of a d.c. motor, in some countries and in some cases provisions have been enacted on safety considerations prescribing that the elevator motor shall be electrically braked, whereby it will operate as a generator. This is usually accomplished by connecting a separate braking resistance across the motor during emergency braking and at the same time interrupting the power supply to the motor. In other cases, the entire braking takes place by the aid of a mechanical brake exclusively. By braking in this manner, the travelling velocity of the elevator will naturally also be retarded independently of external factors.
As a rule, the connecting of the braking resistance is so accomplished that during emergency braking the power supply of the motor is automatically cut off and the brake resistance is connected across the motor. This arrangement, however, suffers from the drawback that a contactor which connects the brake resistance across the motor is necessarily a component of rare quality and high price, owing to the heavy direct currents generated by the motor. The contactor is special particularly for the reason that it is required to carry a heavy current in the deenergized state. Standard contactors, in contrast, carry heavy currents in the energized state.
It is accordingly an object of the present invention to eliminate this drawback.